The present invention relates generally to stents, which are endoprostheses implanted into vessels within the body, such as blood vessels, to support and hold open the vessels, or to secure and support other endoprostheses in the vessels. In particular, the present invention relates to a stent which is longitudinally flexible before and after expansion.
Various stents are known in the art. Typically stents are generally tubular in shape, and are expandable from a relatively small, unexpanded diameter to a larger, expanded diameter. For implantation, the stent is typically mounted on the end of a catheter, with the stent being held on the catheter at its relatively small, unexpanded diameter. By the catheter, the unexpanded stent is directed through the lumen to the intended implantation site. Once the stent is at the intended implantation site, it is expanded, typically either by an internal force, for example by inflating a balloon on the inside of the stent, or by allowing the stent to self-expand, for example by removing a sleeve from around a self-expanding stent, allowing the stent to expand outwardly. In either case, the expanded stent resists the tendency of the vessel to narrow, thereby maintaining the vessel""s patency.
U.S. Pat. No. 5,733,303 to Israel et al. (xe2x80x9c""303xe2x80x9d), which is expressly incorporated by reference, shows a unique stent formed of a tube having a patterned shape which has first and second meander patterns having axes extending in first and second directions. The second meander patterns are intertwined with the first meander patterns to form flexible cells. Stents such as this one are very flexible in their unexpanded state such that they can be tracked easily down tortuous lumens. Upon expansion, these stents provide excellent radial support, stability, and coverage of the vessel wall. These stents are also conformable, in that they adapt to the shape of the vessel wall during implantation.
One feature of stents with a cellular mesh design such as this one, however, is that they have limited longitudinal flexibility after expansion, which may be a disadvantage in particular applications. This limited longitudinal flexibility may cause stress points at the end of the stent and along the length of the stent. Conventional mesh stents like that shown in U.S. Pat. No. 4,733,665 may simply lack longitudinal flexibility, which is illustrated by FIG. 1, a schematic diagram of a conventional stent 202 in a curved vessel 204.
To implant a stent, it maybe delivered to a desired site by a balloon catheter when the stent is in an unexpanded state. The balloon catheter is then inflated to expand the stent, affixing the stent into place. Due to the high inflation pressures of the balloonxe2x80x94up to 20 atmxe2x80x94the balloon causes the curved vessel 204 and even a longitudinally flexible stent to straighten when it is inflated. If the stent, because of the configuration of its mesh is or becomes relatively rigid after expansion, then the stent remains or tends to remain in the same or substantially the same shape after deflation of the balloon. However, the artery attempts to return to its natural curve (indicated by dashed lines) in FIG. 1 with reference to a conventional mesh stent. The mismatch between the natural curve of the artery and the straightened section of the artery with a stent may cause points of stress concentration 206 at the ends of the stent and stress along the entire stent length. The coronary vasculature can impose additional stress on stents because the coronary vasculature moves relatively significant amounts with each heartbeat. For illustration purposes, the difference between the curve of the vessel and the straightened stent has been exaggerated in FIG. 1.
U.S. Pat. No. 5,807,404 to Richter, which is expressly incorporated by reference, shows another stent which is especially suited for implantation into curved arterial portions or osteal regions. This stent can include sections adjacent the end of the stent with greater bending flexibility than the remaining axial length of the stent. While this modification at the end of the stent alleviates the stress at the end points, it does not eliminate the stress along the entire length of the stent.
Various stents are known that retain longitudinal flexibility after expansion. For example, U.S. Pat. Nos. 4,886,062 and 5,133,732 to Wiktor (xe2x80x9cthe Wiktor ""062 and ""732 patentsxe2x80x9d) show various stents formed of wire wherein the wire is initially formed into a band of zig-zags forming a serpentine pattern, and then the zig-zag band is coiled into a helical stent. The stents are expanded by an internal force, for example by inflating a balloon.
The coiled zig-zag stents that are illustrated in FIGS. 1 through 6 of the Wiktor ""062 and ""732 patents are longitudinally flexible both in the expanded and unexpanded condition such that they can be tracked easily down tortuous lumens and such that they conform relatively closely to the compliance of the vessel after deployment. While these stents are flexible, they also have relatively unstable support after expansion. Furthermore, these stents leave large portions of the vessel wall uncovered, allowing tissue and plaque prolapse into the lumen of the vessel.
Thus, it is desired to have a stent which exhibits longitudinal flexibility before expansion such that it can easily be tracked down tortuous lumens and longitudinal flexibility after expansion such that it can comply with the vessel""s natural flexibility and curvature while still providing continuous, stable coverage of a vessel wall that will minimize tissue sag into the lumen.
Embodiments of the present invention provide a stent that is longitudinally flexible before expansion so that it can easily be tracked down tortuous vessels and remains longitudinally flexible after expansion such that it will substantially eliminate any stress points by complying with the vessel""s flexibility and assuming the natural curve of the vessel.
Embodiments of the present invention also to provide a stent that is longitudinally flexible after delivery such that it flexes during the cycles of the heartbeat to reduce cyclic stress at the ends of the stent and along the stent. In some embodiments, the stress experienced during such flexes is below the elastic limit of the material and thus, a very high number of flexes, without fatigue is possible
In addition, embodiments of the present invention provide a stent with a closed cell pattern such that it provides good coverage and support to a vessel wall after expansion.
A stent according to the invention retains the longitudinal flexibility associated with the ""303 cellular stent in its unexpanded state, and has increased longitudinal flexibility in the expanded state. The stent does so without sacrificing scaffoldingxe2x80x94i.e. coverage of the vessel wallxe2x80x94or radial support.